Router LSA quiz: the results

The Friday’s OSPF quiz has generated numerous answers … unfortunately many of them incorrect. Some readers (probably those that recently attended a Cisco certification exam) thought I was asking a trick question, as I’ve forgotten to include the IP addresses in the sample configuration, which only proves how hard it is to write good bulletproof questions.

Those that assumed the IP addresses would have to be configured on the interfaces made two common errors:

  • Some assumed a type-2 LSA would be generated for the LAN interface. Wrong: type-2 LSA is generated only if needed (there is more than one router attached to the LAN interface).
  • Others thought the router would generate a type-1 LSA per interface. Wrong: an OSPF router generates only a single type-1 LSA per area.

To clarify these issues, I wrote an article in the CT3 wiki documenting how the type-1 (router) LSA describes various interface types and inter-router links.

Read the Type-1 (Router) LSA article in the CT3 wiki

4 comments:

  1. Ivan,

    Thanks for such a wonder document of LSA.

    regards
    shivlu jain

    ReplyDelete
  2. It seems a router will originate a single router-LSA (considering the router is a part of single area) containing a single router-link (stub network; considering only one interface is enabled with ospf) whether or not adjacency is formed.

    However, what is the reason to advertise a stub link when an adjacency is formed? what information is provided to the neighbor router with this stub link? *DONT_KNOW*

    ReplyDelete
  3. Does anything prevent Full adjacency between a broadcast interface on one end and point-to-point configured on the other? If I understand right Standard doesn't say anything on this or even packets don't carry any related information to prevent it.

    ReplyDelete
  4. You need to adjust OSPF timers manually. I'm also not sure how the DR/BDR election would work in this case.

    ReplyDelete

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Ivan Pepelnjak, CCIE#1354, is the chief technology advisor for NIL Data Communications. He has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced technologies since 1990. See his full profile, contact him or follow @ioshints on Twitter.